An important source of xylene in an oil refinery is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (BTX), along with ethylbenzene.
However, the quantity of xylene available from reforming is limited and so recently refineries have also focused on the production of xylene by transalkylation of C9+ aromatic hydrocarbons (or simply “A9+”) with benzene and/or toluene over noble metal-containing zeolite catalysts. By way of example, U.S. Pat. No. 5,030,787 teaches using MCM-22 as the zeolite catalyst for transalkylation. Numerous modifications of transalkylation reactions are described in the prior art, such as U.S. Application Publication No. 2010-0298117 and U.S. application Ser. Nos. 12/973,358 and 12/973,331 and references cited therein.
The present inventors have discovered that when processing higher aromatic hydrocarbon content feeds in a transalkylation reaction systems that chlorides accumulate on the catalyst during the cycle to levels exceeding 500 ppmw. When the catalyst is then regenerated by oxygen burn some of the chlorides are released and can concentrate in the regeneration water that condenses in the effluent product cooler and high pressure separator. These chlorides, as HCl, lower the water condensate pH to levels as low as 2.5 causing potential corrosion of carbon steels and the chloride ions in the aqueous solution can also cause stress corrosion cracking of stainless steels. The source of the chloride impurities (or more generally halide and/or halogen impurities) can be from one or more of the hydrocarbon feed sources, the hydrogen make up feed, or a combination thereof. In particular, without wishing to be bound by theory, it is believed that at least one source of the chlorides comes from the feed component(s) which originate from the naphtha reformer, which uses chlorine (and possibly other halogen species) for catalyst activity maintenance. By way of example, chloride levels in the aromatic feed on the order of 100 ppb or less can result in the levels observed in excess of 500 ppmw on the spent coked catalyst over a 1-2 year cycle if all the chloride in the feed is absorbed. We have also found that higher levels of chlorides on regenerated catalyst can be detrimental to the catalyst's second cycle aging performance.
U.S. Pat. No. 7,154,014 teaches that deactivating contaminants present in typical hydrocarbon feeds, such as chlorides, can be removed with a gamma alumina guard bed prior to contacting with a transalkylation catalyst, particularly those having a solid-acid component such as mordenite, and a metal component such as rhenium. The invention suggests that the feed needs to be heated prior to dechlorination guard bed. The requirement of heat and the showing that gamma alumina (activated and high surface area) is superior to alpha alumina (inactive and low surface area) suggests a catalytic function to the guard bed. See also U.S. Patent Application No. 20070086933 and U.S. Pat. No. 7,307,034.
These solutions do not address all of the problems noted by the present inventors and in addition suggests use of the heat generated in the transalkylation reactor to heat the feed to the guard bed. Such heat is at a premium in chemical and refinery operations and is better used elsewhere. Furthermore the suggestion of a catalytic function of the guard bed could give unpredictable results depending on the source of the feed and attendant impurities.
The present invention solves these problems by removing the chlorides from the feed by removing chlorides from the stream before the feed enters the reactor using a guard bed different from that suggested in the prior art and that can operate at ambient temperature, without the need for heat input.